Air separation

Abstract: A process and apparatus for producing a gaseous oxygen product at a delivery pressure containing a low concentration of heavy impurities in which compressed and purified air is cooled in a main heat exchanger 26 to near its dew point temperatures and then introduced into a double rectification column 28 comprising a high pressure column 30 and a low pressure column 32 in a heat transfer relationship with one another through a condenser-reboiler 40. A liquid oxygen fraction 45 separated in the low pressure column 32 and collects in the sump 42 of the column 32. Partial reboiling of the liquid in the sump 42 results in its concentration of heavy impurities being enhanced relative to the liquid phase flowing into the sump through a downcomer 44. A main product stream is withdrawn from the liquid phase before it reaches the sump and is pumped to a delivery pressure and then vaporized within the main heat exchanger 26. A purge stream of liquid oxygen from the sump is removed so that the impurity concentration level within the main liquid oxygen product stream does not reach its solubility limit.

Description: The present invention relates to a process and apparatus for producing a gaseous oxygen product at a delivery pressure by rectifying air. In cryogenic air separation plants that produce gaseous oxygen at a delivery pressure by vaporizing pumped liquid oxygen within a main heat exchanger, heavy impurities such as carbon dioxide and hydrocarbons can exceed their solubility limits in the liquid oxygen as it vaporizes. As a result, carbon dioxide contained within the liquid oxygen can solidify to plug heat exchange passageways within the main heat exchanger and hydrocarbons such as acetylene can come out of solution to present a safety hazard. This occurs because the heavy impurities such as carbon dioxide and hydrocarbons have a much lower vapour pressure than oxygen and as such, tend to concentrate in liquid oxygen being produced within the air separation plant. When the liquid oxygen is raised to a higher pressure by pumping and then vaporized by being heated within the main heat exchanger of the air separation plant, the resulting increase in vaporization temperature increases the vapour pressures of the heavy impurities to a degree greater than the oxygen vapour pressure increase and hence, the heavy impurities vaporize before the liquid oxygen is fully vaporized. The heavy impurities may then freeze on heat exchange surfaces within the main heat exchanger thus deleteriously affecting the performance of the heat exchanger. EP-A-0 024 962 relates to an air separation process of the internal kind in which the main oxygen product stream is taken from the sump of the low pressure column. Both JP-A-3099190 and DE-A-3 016 317 disclose double column air separation processes not of the internal compression kind in which oxygen product is taken from above the bottom stage of the low pressure column so as to diminish its content of hydrocarbon impurity. JP-A-3099190 shows that a liquid oxygen stream 16 is withdrawn from the sump of the low pressure column and the preamble to claims 1, 2 and 7 below is based thereupon. As will be discussed, the present invention provides a process and apparatus for the separation of air to produce a gaseous oxygen product at a delivery pressure with a low level of heavy impurity concentration and without delivering the product at a higher than necessary delivery pressure. The present invention provides a process for producing a gaseous oxygen product at a delivery pressure and so as to contain a low concentration of heavy impurities. As used herein, heavy impurities include carbon dioxide and such hydrocarbons as acetylene. These heavy impurities are but examples of those that create problems in air separation plants. Carbon dioxide can plug up heat exchanger tubes and acetylene can present an explosion hazard during the production of oxygen. In its broadest aspect, the present invention provides a process for producing a gaseous oxygen product at a delivery pressure and so as to contain a low concentration of heavy impurities air comprising the steps of cooling a stream of compressed air in a main heat exchanger to a temperature suitable for its separation by rectification, fractionating the air (or a fluid mixture separated therefrom) in a rectification column and thereby obtaining liquid oxygen, reboiling a part of said liquid oxygen so as to create a volume of residual liquid oxygen relatively rich in heavy impurities, withdrawing a purge stream of said residual liquid oxygen, withdrawing a major stream of oxygen, relatively lean in heavy impurities, from the said rectification column, characterised by taking the major stream in liquid state from upstream of the reboiling, pumping the purge stream to a sufficiently high pressure level that on vaporisation the heavy impurities vaporise substantially with the liquid oxygen contained within the purge stream pumping the major stream to a delivery pressure, and vaporising the major stream and the purge stream in the main heat exchanger. The invention also provides apparatus for separating air, comprising a main heat exchanger for cooling a stream of compressed air to a temperature suitable for its separation by rectification, a rectification column for fractionating the air (or a fluid mixture separated therefrom) having a sump associated therewith for collecting liquid oxygen, a reboiler associated with the sump for reboiling a part of said liquid oxygen so as to create, in use, a volume of residual liquid oxygen relatively rich in heavy impurities, characterised by a first pump for withdrawing and pressurising a purge stream of said liquid oxygen, and a second pump for withdrawing a major stream of liquid oxygen, relatively rich in heavy impurities, from upstream of the said sump, wherein both pumps have an outlet communicating with vaporising passages in the main heat exchanger and the apparatus is operable such that the first pump is able to raise the pressure of the purge stream to a level at which the heavy impurities therein vaporise with the oxygen in the main heat exchanger. The process and apparatus according to the present invention are able to make use of a wide range of different arrangements of columns, reboilers and condensers in order to effect the separation of the oxygen. One preferred embodiment of the process according to the invention will now be described. Air is compressed and, after removal of the heat of compression, is purified. The air is cooled within a main heat exchanger to a temperature suitable for its rectification. The air is then introduced into a double rectification column so that the air is rectified. The double rectification column includes high and low pressure columns operatively associated with one another in a heat transfer relationship by provision of a condenser-reboiler having a sump. Each of the high and low pressure columns have contacting elements for contacting an ascending vapour phase having an ever-increasing nitrogen concentration as the vapour phase ascends with a descending liquid phase having an ever-increasing oxygen and heavy impurity concentration as the liquid phase descends. In the low pressure column, liquid oxygen having a high concentration of heavy impurities collects in the sump of the condenser-reboiler. The liquid phase flowing into the sump, though, has a low concentration of the heavy impurities. Refrigeration is introduced into the process so that heat balance within the process is maintained. A major liquid oxygen stream is withdrawn from the low pressure column, which is composed of the liquid phase flowing to the sump associated with the condenser-reboiler. The major liquid oxygen stream is pumped to a delivery pressure and is then vaporized within the main heat exchanger to produce the gaseous oxygen product. A purge liquid oxygen stream, composed of the liquid oxygen collected in the sump of the condenser-reboiler, is withdrawn from the low pressure column such that the heavy impurities do not concentrate in the liquid oxygen at a level above their solubility limit. In another preferred aspect, the present invention provides an apparatus for rectifying air to produce a gaseous oxygen product at a delivery pressure and so as to contain a low concentration of heavy impurities. The apparatus comprises means for compressing and for purifying the air. A main heat exchanger communicates with the compressing and purifying means for cooling the air to a temperature suitable for its rectification against vaporizing a pumped liquid oxygen stream forming the gaseous oxygen product. A means is provided for introducing refrigeration into the apparatus and thereby maintaining the apparatus in heat balance. A double column air separation unit is provided having high and low pressure columns operatively associated with one another in a heat transfer relationship by provision of a condenser-reboiler having a sump. Each of the high and low pressure columns have contacting elements for contacting an ascending vapour phase having an ever-increasing nitrogen concentration as the vapour phase ascends with a descending liquid phase having an ever-increasing oxygen and heavy impurity concentration as the liquid phase descends. In the low pressure column, liquid oxygen having a high concentration of the heavy impurities collects in the sump of the condenser-reboiler and the liquid phase flowing into the sump has a low concentration of the heavy impurities. A pump is connected between the main heat exchange means and the low pressure column such that the liquid oxygen composed of the liquid phase flowing to the sump is pumped to the delivery pressure and thereby forms the liquid oxygen stream. A means is provided for withdrawing the liquid oxygen collected in the sump of the condenser-reboiler such that the heavy impurities do not concentrate in the liquid oxygen at a level above their solubility limit. Since heavy impurity concentration within the liquid oxygen being vaporized within the main heat exchanger is low enough to begin with, vaporization of the heavy impurities within the main heat exchanger does not contribute to any equipment or safety hazards. It should be noted that the term "main heat exchanger" as used herein and in the claims does not necessarily mean a single (plate fin heat) exchanger. A "main heat exchanger" as would be known to those skilled in the art, could be made of several units working in parallel to cool and warm streams. The use of high and low pressure heat exchangers is conventional in the art. Additionally, the terms "fully cooled" and "fully warmed" as used herein and in the claims main cooled to rectification temperature and warmed to ambient, respectively. The term "partially" in the context of being partially warmed or cooled as used herein and in the claims indicates the warming or cooling to a temperature between fully warmed and cooled. The process and apparatus according to the invention will now be described with reference to the accompanying drawing which is a schematic flow diagram of an air separation plant. With reference to the drawing, an apparatus 10 for carrying out a method in accordance with the present invention is illustrated. In apparatus 10, an air stream 12 after having been filtered is compressed by a main compressor 14. Thereafter, heat of compression is removed by a first aftercooler 16 and the air is purified by an air purification unit 18 in which carbon dioxide, moisture and hydrocarbons are substantially removed from the air. As will be discussed, a certain amount of carbon dioxide and other heavy impurities such as hydrocarbons remain in the air. Apparatus 10 is designed to deliver a gaseous oxygen at a delivery pressure. This is accomplished by pumping liquid oxygen to the requisite pressure. In order to vaporize the oxygen product, the air is further compressed in a high pressure air compressor 20 to form a further compressed air stream 22. After having been further compressed, the heat of compression is removed from further compressed air stream 22 by a second aftercooler 24. Further compressed air stream 22 is then cooled in a main heat exchanger 26 to a temperature suitable for its rectification, which in practice would be at or near its dew point temperature. The further compression of the air is necessary to vaporize a highly pressurized oxygen product. It is to be noted that the present invention has equal applicability to an air separation plant in which the product is delivered at a lower pressure. In such case the air would not have to be further compressed. Air stream 24 is then introduced into a double rectification column 28 having high and low pressure columns 30 and 32 after being suitably reduced to high and low pressure column pressures by Joule-Thomson valves 34 and 35. Each of the high and low pressure columns 30 and 32 are provided with contacting elements, designated by reference numeral 36 for the high pressure column and 38 for low pressure column 32. Contacting elements 36 and 38 (sieve plates, trays, structured or random packings) are utilized to contact descending vapour and liquid phases. In each column, as the vapour phase ascends through the packing elements it becomes increasingly more concentrated in nitrogen as it ascends and the liquid phase becomes increasingly more concentrated in oxygen as t descends an oxygen-enriched liquid or crude liquid oxygen fraction is obtained at the bottom of the column, and a nitrogen-enriched vapour fraction is obtained at the top. The nitrogen-enriched vapour is condensed to form liquid nitrogen by a condenser-reboiler 40 having a sump 42 in low pressure column 32. In low pressure column 32, as the liquid phase becomes more concentrated in the less volatile oxygen, it also becomes more concentrated in the heavy impurities. These heavy impurities further concentrate in the liquid oxygen that collects within sump 42 of condenser-reboiler 40 since the action of the reboiler is to vaporise other components of the liquid in preference to the heavy impurities. The liquid oxygen is vaporized by condenser-reboiler 40 against the condensation of the nitrogen-enriched vapour in high pressure column 30. In the illustrated embodiment, trays are used and liquid descends from tray to tray by downcomers of which downcomer 44 is illustrated. The liquid phase passing from downcomer 44 prior to the time it reaches sump 42 contains significantly a significantly lower concentration of the heavy impurities than the liquid oxygen collected in sump 42 of condenser-reboiler 40. The liquid nitrogen from condenser-reboiler 40 is used to reflux high pressure column 30 by provision of a stream 46 and low pressure column 42 by provision of a stream 48. Stream 48 is subcooled within a subcooler 50, reduced to the pressure of low pressure column 32 by provision of a Joule-Thomson valve 54 and introduced into low pressure column 32. An air stream 56, representing a portion of air stream 22, is also subcooled in subcooler 50 prior to its expansion and introduction into low pressure column 32. A crude liquid oxygen stream 60, is withdrawn from high pressure column 30, subcooled in subcooler 50, reduced in pressure to that of the low pressure column by a Joule-Thomson valve 62 and introduced into low pressure column 32 for further refinement. A nitrogen vapour stream 64 composed of the nitrogen vapour produced within low pressure column 32 is partially warmed in subcooler 50 by heat transfer with nitrogen reflux stream 48, air stream 56, and crude liquid oxygen stream 60 in order to subcool the same. Waste nitrogen stream 64 then passes through main heat exchanger 26 where it fully warms and where, preferably, it is used in regenerating air purification unit 18. It can also, in whole or part, be expelled from the system. In order to keep apparatus 10 in heat balance, refrigeration is supplied through air expansion. To this end, air stream 12 is divided into first and second subsidiary streams 68 and 70. First subsidiary stream 68 is compressed by high pressure air compressor 20. The second subsidiary stream 70 after having been partially cooled is divided into first and second partial streams 72 and 74 by provision of an intermediate outlet of main heat exchanger 26. First partial stream 72 is expanded by a turboexpander 76 which performs expansion work which is either discharged or used in compression of the air to form a turboexpanded stream 78 which is introduced into low pressure column 32 to supply refrigeration and thereby maintain apparatus 10 in heat balance. It is understood that the present invention would have equal applicability to a nitrogen expansion plant. Second partial stream 74 is fully cooled within main heat exchanger 26 and then, introduced into the bottom of high pressure column 30 for rectification. In order to produce the gaseous oxygen product, the liquid phase flowing to the sump is withdrawn from low pressure column 32 at downcomer 44 as a major liquid oxygen stream 80 which after withdrawal is pumped by a liquid oxygen pump 82 to the delivery pressure. Major liquid oxygen stream 80 is then vaporized within main heat exchanger 26. It is to be noted here that in case of structured packing, a major liquid oxygen stream would be withdrawn from a liquid collector at the same location as downcomer 44. In order to prevent the heavy impurities from climbing above their solubility limits in the liquid oxygen by interfering with the air separation or creating a safety hazard, liquid oxygen is removed from sump 42 of condenser-reboiler 40 as a purge liquid oxygen stream 84 which is pumped to a higher pressure than the delivery pressure by a pump 86. Purge liquid oxygen stream 84 then is vaporized within main heat exchanger 26. The high pressure pumping of purge liquid oxygen stream 84 guarantees that the impurities will vaporize with the oxygen within main heat exchanger 26. The pumped liquid oxygen stream 80 after vaporization becomes the main gaseous oxygen product and the pumped purge liquid oxygen stream 84 becomes a minor gaseous oxygen product. The major and minor gaseous oxygen products can be combined and delivered to the customer. However, since in a properly designed case, the minor oxygen product will amount to about 5% of the liquid oxygen product, it can also simply be purged from apparatus 10 or stored as a liquid (without pumping and vaporization) for some other use. EXAMPLE The following is a calculated example of the operation of apparatus 10. In apparatus 10, high pressure column is provided with 30 theoretical stages. Second partial stream 74 from main heat exchanger 26 enters main heat exchanger below stage 30 and a portion of the compressed air stream 24 is introduced as liquid into stage 24. Stream 48 is withdrawn from high pressure column 30 at the top stage thereof. The low pressure column 32 has 40 theoretical stages and stream 48 is subcooled in subcooler 50 and introduced into top stage, stage 1, of low pressure column 32. Crude liquid oxygen 60 after having been subcooled in subcooler 50 is introduced onto stage 25. The balance the further compressed air stream 22, namely air stream 56, after having been subcooled in subcooler 50, is introduced onto stage 15 of low pressure column 32. Turboexpanded stream 78 is introduced into low pressure column 32 above stage 28. Stream Flow(Nm 3 /min) Temp(°C) Pressure bara % O 2 Air stream 12 after air pre-purification unit 18 1000 26.7 5.52 21 Further compressed air stream 22 300 26.7 10.34 21 Second subsidiary stream 70 75 26.7 5.52 21 Second partial stream 74 625 -173.3 5.45 21 Portion of further compressed air stream 22 introduced into high pressure column 30 75 -173.3 10.2 21 First partial stream 72 75 -101.1 5.45 21 Portion of further compressed stream 22 introduced into low pressure column 30 75 -147.7 1.48 21 Stream 48 before subcooling 300 -178.2 5.38 0.0 Crude oxygen liquid stream 60 before subcooling 400 -174.0 5.45 36.7 Air stream 56 before subcooling 225 -173.3 10.2 21 Main liquid oxygen stream 80 (before pumping) 210 -179.7 1.50 95.0 Purge liquid oxygen stream 84 before pumping 10 -179.3 1.50 97.1 Main O 2 product 210 24.3 3.66 95.0 Minor O 2 product 10 24.3 10.3 97.1 Waste nitrogen stream 64 after fully warmed within main heat exchanger 26 780 24.3 1.27 0.06 It is to be noted that main oxygen product has a CO 2 concentration of about 0.058 vpm and purge oxygen product has a CO 2 concentration of about 2.5 vpm. These conditions under the scope of the present invention have the following effect when air stream 12, after having been purified in air prepurification unit 18 contains about 0.037 vpm CO 2 . In a conventional plant the liquid oxygen product from the low pressure column will contain about 0.17 vpm of dissolved carbon dioxide. The liquid oxygen would have to be pumped to at least 5.31 bara before vaporizing in order to prevent precipitation of CO 2 in main heat exchanger 26. This would require further compressed air stream 22 to be compressed to greater than 10.34 bara. Most of the liquid oxygen is pumped to only 3.79 bara and only a small amount to 10.4 bara (purge stream 84). A further compressed air stream 22 of 10.34 bara is adequate to ensure vaporization of both major and purge liquid oxygen streams 80 to 84 in the main heat exchanger without carbon dioxide freeze out and to keep the carbon dioxide in condenser-reboiler 40 below its solubility limit.

Claims: A process for producing a gaseous oxygen product at a delivery pressure and so as to contain a low concentration of heavy impurities comprising the steps of cooling a stream of compressed air (70) in a main heat exchanger (26) to a temperature suitable for its separation by rectification, fractionating the air (or a fluid mixture separated therefrom) in a rectification column (28) and thereby obtaining liquid oxygen, reboiling a part of said liquid oxygen so as to create a volume of residual liquid oxygen relatively rich in the heavy impurities, withdrawing a purge stream (84) of said residual liquid oxygen, withdrawing a major stream (80) of oxygen, relatively lean in heavy impurities, from the said rectification column (28), characterised by taking the major stream in liquid state from upstream of the reboiling, pumping the purge stream (84) to a sufficiently high pressure level that on vaporisation the heavy impurities vaporise substantially with the liquid oxygen contained within the purge stream (80), pumping the major stream (80) to a delivery pressure, and vaporising the major stream (80) and the purge stream (84) in the main heat exchanger (26). A process for producing a gaseous oxygen product at a delivery pressure and so as to contain a low concentration of heavy impurities, said process comprising: compressing a stream (12) of the air, removing heat of compression from the compressed air stream, and purifying the compressed air stream; cooling at least part of (26, 70) the compressed air stream (12) within a main heat exchanger (26) to a temperature suitable for its rectification; introducing the compressed air stream into a double rectification column (28) so that the air is rectified, said double rectification column (28) including high and low pressure columns (30 and 32) operatively associated with one another in a heat transfer relationship by provision of a condenser-reboiler (40) having a sump (42), each of the high and low pressure columns (30 and 32) having contacting elements (36 and 38) for contacting an ascending vapour phase having an ever increasing nitrogen concentration as the vapour phase ascends with a descending liquid phase having an ever increasing oxygen and heavy impurity concentrations as the liquid phase descends such that, in the low pressure column (32), liquid oxygen having a high concentration of the heavy impurities collects in the sump (42) of the condenser-reboiler (40) and the liquid phase flowing to the sump (42) has a low concentration of the heavy impurities; introducing refrigeration into the process so that heat balance within the process is maintained; withdrawing a major oxygen stream (80) from the low pressure column (32); withdrawing a purge liquid oxygen stream (84) from the low pressure column (32) composed of the liquid oxygen collected in the sump (42) of the condenser-reboiler (40) such that the heavy impurities do not concentrate in the liquid oxygen at a level above their solubility limit; characterised by: taking the major oxygen stream (80) from the liquid phase flowing to the sump (42) of the condenser-reboiler, pumping it to the delivery pressure and vaporizing said liquid oxygen stream within the main heat exchanger (26) to produce said gaseous oxygen product, pumping the purge liquid oxygen stream (84) to a sufficiently high pressure level that the heavy impurities will vaporize substantially with the liquid oxygen contained within said purge liquid oxygen stream (84); and vaporizing the purge liquid oxygen stream within the main heat exchanger (26). A process according to Claim 2, further comprising: further compressing a portion of the compressed air stream (12) to form a further compressed air stream (22); cooling the further compressed air stream (22) within the main heat exchanger (26) to the temperature suitable for its rectification; and introducing the further compressed air stream (22) into the double rectification column (28). A process according to claim 3, wherein a part of the further compressed air stream (22) is reduced in pressure and introduced into the high pressure column (30) and another part of the further compressed air stream (22) is reduced in pressure and introduced into the low pressure column (32). A process according to claim 4, wherein: the descending liquid phase within the high pressure column (30) collects at the bottom thereof as an oxygen enriched liquid and the ascending vapour phase at the top thereof as nitrogen-enriched vapour; the nitrogen enriched vapour is condensed by indirect heat exchange with evaporating liquid oxygen collected in the sump (42) of the low pressure column (30); a nitrogen vapour fraction is formed at the top of the low pressure column; a stream (60) of the oxygen-enriched liquid is withdrawn from the high pressure column (30), subcooled, reduced to the low pressure column (32) pressure and introduced into the low pressure column (32) for further rectification; a stream of the condensed nitrogen enriched vapour is withdrawn from the condenser-reboiler and divided into two liquid nitrogen partial streams (46, 48), one (46) of said two liquid nitrogen partial streams (46, 48) is supplied to the high pressure column (30) as reflux and the other (48) of the two liquid nitrogen partial streams (46, 48) is subcooled, reduced to the low pressure column (32) pressure, and introduced into the low pressure column (32) as reflux; and a waste nitrogen stream (64) composed of the nitrogen vapour fraction separated in the low pressure column (32) is withdrawn from the low pressure column (32), partially warmed against oxygen-enriched liquid being sub-cooled and the other (48) of the two liquid nitrogen partial streams (46, 48), and is fully warmed in the main heat exchanger (26). A process according to any one of claims 2 to 5, wherein: the contacting elements (36, 38) comprise trays having downcomers; the major liquid oxygen stream (80) is withdrawn from the downcomer (44) associated with the lowermost tray in the low pressure column (32). Apparatus for separating air, comprising a main heat exchanger (26) for cooling a stream of compressed air to a temperature suitable for its separation by rectification, a rectification column (28) for fractionating the air (or a fluid mixture separated therefrom) having a sump (42) associated therewith for collecting liquid oxygen, a reboiler (40) associated with the sump (42) for reboiling a part of said liquid oxygen so as to create, in use, a volume of residual liquid oxygen relatively rich in heavy impurities, characterised by a first pump (86) for withdrawing and pressurising a purge stream of said liquid oxygen, and a second pump (82) for withdrawing a major stream of liquid oxygen, relatively rich in heavy impurities, from upstream of the said sump, wherein both pumps (82, 86) have an outlet communicating with vaporising passages in the main heat exchanger and the apparatus is operable such that the first pump (86) is able to raise the pressure of the purge stream to a level at which the heavy impurities therein vaporise with the oxygen in the main heat exchanger (26).

Inventors: MOSTELLO ROBERT A, MOSTELLO, ROBERT A.
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1994
Main classification: F25J3
IPC codes: F25J3